If I were you, I'd use oxyacetylene welding and brass rod to join carbon steel to copper. I've done that many times with excellent results.
Giovanni S. Crisi
Sao Paulo - Brazil

It can certainly be done. I would suggest consulting the AWS Handbook Volume 3 for info. Any welding to copper will require high preheats (700 °F +, and will not look pretty in my experience). Filler metals could be AWS ERCuAl-A2 (aluminum bronze) or ERCu. One producer of these metals that I have come across is Wisconson Wire Works, www.wisconsinwireworks.com. They may be able to help you based on thieir experience with similar applications.

Regards.